The United Nations is to set up a dedicated investigations unit in Geneva to examine the legality of drone attacks in cases where civilians are killed in so-called ‘targeted’ counter-terrorism operations.

The announcement was made by Ben Emmerson QC, a UN special rapporteur, in a speech to Harvard law school in which he condemned secret rendition and waterboarding as crimes under international law.

His forthright comments, directed at both US presidential candidates, will be seen as an explicit challenge to the prevailing US ideology of the global war on terror.

Earlier this summer, Emmerson, who monitors counter-terrorism for the UN, called for effective investigations into drone attacks. Some US drone strikes in Pakistan – where those helping victims of earlier attacks or attending funerals were killed – may amount to war crimes, Emmerson warned.